Advice:How to tell if she's gay?

I've been living in confusion for the past 3 years and i finally accepted my bisexuality towards the end of last year.the thing is i'm getting overly frustrated on how to make a move on girls because i think i have a faulty gaydar.i can't seem to tell if a girl is gay or not.it's really hard to do so.i've been exploring my sexuality further since i broke up with my boyriend 2 months ago and even with that,i can't seem to find anyone.at all.i've had previous girl crushes and all of them came and went because i didn't dare a make a move.it's getting really upsetting for me.anyhows,there's this girl in my writing class and she's hot and tomboyish at the same time.i really like her but the thing is i don't even dare look at her or talk to her.Gosh.please help.